Device comprising an electric high vacuum discharge tube provided with at least two electrodes not destined for emission, and discharge tube for such a device

ABSTRACT

The effects of high field emission upon electrodes and screens in high vacuum tubes or switches is reduced by coating copper electrodes with a thin layer of titanium or zirconium. As a result of this, the allowable field strength is increased.

United States Patent Van Oostrom [451 Sept. 25, 1973 DEVICE COMPRISING AN ELECTRIC HIGH VACUUM DISCHARGE TUBE PROVIDED WITH AT LEAST TWO ELECTRODES NOT DESTINED FOR EMISSION, AND DISCHARGE TUBE FOR SUCH A DEVICE Inventor: Antonius Gerardus Johannes Van Oostrom, Emmasingel, Eindhoven, Netherlands Assignee: U.S. Philips Corporation, New York,

Filed: June 18, 1971 Appl. No.: 154,348

Foreign Application Priority Data June 30, 1970 Netherlands 7009601 US. Cl. 313/311, 313/107, 200/144 B Int. Cl H01j 19/30, I-lOlh 9/30 Field of Search 313/311, 106, 107;

200/144 R, 144 B, 166 C [56] References Cited UNITED STATES PATENTS 2,846,609 7/1958 Espersen 313/311 X 2,734,003 2/1956 Alpert 313/311 X FOREIGN PATENTS OR APPLICATIONS 6,401,843 2/1964 Netherlands 313/311 Primary ExaminerRudolph V. Rolinec Assistant ExaminerMarvin Nussbaum Att0rneyFrank R. Trifari 5 7 ABSTRACT The effects of high field emission upon electrodes and screens in high vacuum tubes or switches is reduced by coating copper electrodes with a thin layer of titanium or zirconium. As a result of this, the allowable field strength is increased.

2 Claims, 5 Drawing Figures ff/r/l///I//I PATENIEDsErzsma Fig.2

INVENTOR. ANTONIUS G.J. VAN OOSTROM f w Q. R.

AGENT PAIENIEU 2 9 3 SHEET 2 BF 3 INVENTOR, VAN 0O STROM ANTONIUS G.J.

LQMA

AGENT PAIENIEDSEPZSIBIBI SHEET 30F 3 Fig.&

- INVENTOR.

ANTONIUS G.J. VAN OOSTROM iuwe AGENT DEVICE COMPRISING AN ELECTRIC IIIGII VACUUM DISCHARGE TUBE PROVIDED WITH AT LEAST TWO ELECTRODES NOT DESTINED FOR EMISSION, AND DISCHARGE TUBE FOR SUCH A DEVICE The invention relates to a device comprising an electric high vacuum discharge tube provided with at least two non-emitting electrodes, between which during operation of the device a high field strength prevails as a result of a voltage difference, direct voltage or low frequency voltage present between said electrodes. The

invention furthermore relates to an electric discharge tube for such a device. High electric field strength is to be understood to mean in this connection a macroscopic field strength of more than kvolt/cm resulting from an applied direct voltage difference or a low frequency alternating voltage difference. The macroscopic field strength follows from the applied voltage difference and the geometry of the arrangement without taking surface structure into account.

Besides devices employing high vacuum tubes, the invention also relates to a device having vacuum switches in which, in addition to contacts, screen electrodes are present.

With field strengths higher than the above stated value, there is a danger of breakdown in a high vacuum discharge tube. Depending upon the value of the applied voltage and the internal resistance of the voltage source, a breakdown may result in damage to the electrodes and even to the entire tube.

One of the causes by which a breakdown can be introduced in a vacuum, is the detachment of microparticles of electrodes or enveloping tube wall under the infiuence of a strong electric field. This cause can be substantially eliminated by careful processing during the manufacture of electrodes and other components as well as their assembly with regard to the absence of dust and smoothly finished surfaces. Moreover, during the actuation of the device, the voltage between the electrodes is increased gradually to restrict the occurrence of discharges if any in intensity.

Another cause of vacuum breakdown which can never be entirely avoided is the field emission from punctiform or wire-shaped projections (protrusions) of very small dimensions on a negative electrode. Such protrusions remain in spite of the very best methods of processing.

The result of said protrusions is that at the free extremity thereof, and also in connection with the small radii of curvature, the field strength is considerably higher than the macroscopic field strength. The field intensification factor, i.e. the ratio of the actual field strength to the point of the protrusions to the macroscopic field strength, for example, is for copper in a quite clean condition and a good vacuum, approximately 100. In the presence of contaminations and in poor vacuum conditions, the field intensification factor may increase to a few hundreds and even above thousand. Under the influence of the high field strengths, field emission occurs from the protrusions. Depending upon the configuration, the material properties, and the form of the applied voltage (continuous or pulsatory) between the electrodes, this field emission results in a temperature increase of one of the electrodes, the negative or the positive one, which locally is so high that the formation of vapour occurs. The ionization formed in said vapour results in breakdown.

It is the object of the invention to provide measures by which the breakdowns introduced by field emission form punctiform or wire-shaped protrusions occur less easily so that higher macroscopic field strengths can be permitted.

According to the invention, in a device employing an electric high vacuum discharge tube provided with at least two non-emitting electrodes and between which during operation of the device a high field strength occurs, the electrodes consist of copper having a closed layer of titanium or zirconium in a thickness of from 10 to 50 AU on at least those parts where a high field strength occurs. As a result of this it is achieved that the admissible field strength can amount to 20 kvolt/mm and more, as a result of which inter alia more compact constructions are possible. The invention is based on the recognition of the fact that in the protrusions formed from copper the temperature increase as a result of the field emission current (Joule heat) is as small as possible, because both the electric and the thermal conductivity of copper are high. Because, however, the surface consists of titanium or zirconium, the vapour pressure is lower than of copper while the thin titanium layer or zirconium layer has no influence on the electric and thermal conductivity. Gases, if any, released by the heating are also readily gettered, at least in none too large quantities. Similar considerations hold good for the positive electrode.

In order to obtain a closed layer of titanium or zirconium, said layer is preferably provided by vapour deposition.

It is to be noted that it is known from U.S. Pat. Nos. 2,955,229 and 3,252,034 to cover the electrodes which bound the inter-action gaps in klystron cavities with a layer of titanium. The object of this is to reduce the secondary emission of said electrodes. The secondary emission occurring may cause damage either to the electrodes themselves or to the ceramic windows in the cavities. During covering the electrodes with titanium, for which in the last-mentioned patent specification a thickness of approximately lOOOAU is stated, ceramic windows, if any, are also covered with a non-cohering layer in a thickness of [00 AU. Since the phenomenon described here is based on secondary emission in high frequency electric fields, it may not be derived from this to apply a considerably thinner coating of titanium or zirconium also in those constructions in which the secondary emission can play no part.

In order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a measuring arrangement FIG. 2 shows diagrammatically a part of an electron microscope.

FIG. 3 is a cross-sectional view through an X-ray tube FIG. 4 is a cross-sectional view through the electron gun of a display tube,

FIG. 5 is a cross-sectional view through a vacuum switch.

Referring now to FIG. 1, two copper electrodes 2 and 3 covered with a layer of titanium 4 and 5, respectively, are present inside the glass tube 1. The diameter of the electrodes is 18 mm and the mutual distance is 0.25mm. The thickness of the titanium layers is 40 AU. With a vacuum of 10" Torr, the current with a voltage difference of 5 kVolt between the electrodes was less than A. This high insulation value was maintained for several weeks.

In FIG. 2, reference numeral 20 denotes the V- shaped thermionic cathode of an electron microscope, 21 is the grid and 22 the accelerating anode. The object is denoted by 23 and the fluorescent screen by 24.

A number of electromagnetic lenses is denoted by 25 to 28. The grid 21 and the anode 22 are coated on the sides facing each other with a layer of zirconium 29 of 30 AU denoted in broken lines. With a distance of 5 mm between 21 and 22, the voltage difference may be more than 125 k.volt.

The X-ray tube shown in FIG. 3 consists of a glass envelope 30 in which is arranged a cathode 31, surrounded by a cathode screen 32. A soft magnetic anode tube 33 is sealed to the glass by means of a wider tube 34 from a sealing alloy. Said soft magnetic tube having an eccentric thickness distribution serves to concentrate the electron beam in the center 36 of the tungsten anode 35 by means of a magnetic system (not shown). The radiation can emanate through a thin beryllium window 37. The screening beaker 32 and the anode tube 33 are coated on the closely adjacent parts with a layer 38 of titanium of 50 AU thickness denoted by broken lines on a heavier layer of copper. The voltage of the tube is 150 kV with a distance between 32 and 33 of 7 mm.

In FIG. 4, the electron gun of a cathode-ray tube for picture display is constituted by a cathode having a surface 40, a first grid 41, a second grid 42, an anode 44 comprising a spout 43, and lens electrodes 45 and 46. The distance between the first and the second grid is 0.25 mm. The first grid is at -l75 volt relative to the cathode, the second grid is at +3,000 volt. The anode 44 is +25 kV relative to the cathode, the electrode 45 +7,200 volt and the electrode 46 again at +25 k.volt. The distance between the spout 43 and the grid 42 is smaller than 1 mm, the mutual distances between the electrodes 44, 45 and 46 are 1 mm. In all places where high field strengths occur, the titanium layer 47 shown in broken lines is provided in a thickness of 20 AU. In addition to the fact that as a result of this measure the field strength between the electrodes 41, 42 and the spout 43 can be increased, the narrowing of the gaps between the electrodes 44 to 46 has the advantage that insulating components, for example, supporting rods and the wall of the tube near said gaps which are charged in an uncontrollable manner have less influence on the beam.

In FIG. 5, the vacuum switch consists of a two-part ceramic housing 50 with terminating plates 51 and 52 of copper. Electrodes 53 and 54 are secured to the plates by means of a bellows 55 and a rod,respectively. Three screening electrodes 56, 57 and 58 screen the wall 50 from sputtering material. Coming from the electrodes. The closely adjacent edges of the screening electrodes are coated with the titanium layer 59 of 50 AU denoted by broken lines. The distances are 5 mm and the voltage of the opened switch kV eff 50 Hz.

What is claimed is:

1. An electrode system comprising a plurality of nonemitting members, and means for producing high intensity electrical fields between said members, said members essentially consisting of copper having a metal coating selected from the group consisting of titanium and zirconium, said coating having a thickness of 10 to 50 angstrom units.

2. An electron discharge tube comprising an evacuated envelope, a plurality of non-emitting electrodes closely spaced from each other, and means for applying potentials to said electrodes thereby producing high intensity electric fields between said electrodes, said electrodes essentially consisting of copper having a coating, varying in thickness between 10 and 50 angstron units selected from the group consisting of titanium and zirconium. 

2. An electron discharge tube comprising an evacuated envelope, a plurality of non-emitting electrodes closely spaced from each other, and means for applying potentials to said electrodes thereby producing high intensity electric fields between said electrodes, said electrodes essentially consisting of copper having a coating, varying in thickness between 10 and 50 angstron units selected from the group consisting of titanium and zirconium. 